build: fix external plugin build

Type: fix
Fixes: 88b2e3682be6303973fc59c3c62141d64a9e10d7

Change-Id: I4b29924246e2cdc307cf238c5cb8624f14317452
Signed-off-by: Benoît Ganne <bganne@cisco.com>
diff --git a/src/cmake/VPPConfig.cmake b/src/cmake/VPPConfig.cmake
index 21da59d..2de35d1 100644
--- a/src/cmake/VPPConfig.cmake
+++ b/src/cmake/VPPConfig.cmake
@@ -35,6 +35,7 @@
 
 set(VPP_RUNTIME_DIR "bin" CACHE STRING "Relative runtime directory path")
 set(VPP_LIBRARY_DIR "lib" CACHE STRING "Relative library directory path")
+set(VPP_BINARY_DIR ${CMAKE_BINARY_DIR}/CMakeFiles)
 
 include(${CMAKE_CURRENT_LIST_DIR}/cpu.cmake)
 include(${CMAKE_CURRENT_LIST_DIR}/api.cmake)